What I found interesting about this article is that it isn't based on a correlation of outcomes and vitamin D like most other results are. It's actually looking at potential mechanisms and vitamin D is coming up as a candidate.

I don't know about air, but ultraviolet light is a pretty good disinfectant.

but patients don't disinfect from the inside when exposed to outdoor air...

ozone too. And it goes inside your nose, throat, lungs.

I'm still waiting for the results from that whole bleach in the lungs idea Trump had. Hoping that will be the cure for all of this.

UV radiation is the disinfectant. Also the vast amount of air makes a dillution of germs and aerosols more likely which is great.

With regards to sunlight exposure, sunbathing in the alps was a go-to tuberculosis treatment prior to having antibiotics.

I would guess "air as disinfectant" is primarily because of its oxygen content. O_2 is actually a pretty nasty molecule like that. Oxidation is typically a fairly tough reaction to reverse or prevent.

Isn't there plenty of O_2 indoors also?

Maybe it's not even a "disinfectant". It's just that you aren't re-breathing stale air with droplets in it, when outside. Assuming, of course, you aren't wearing a mask. :-)